1.Field of the Invention
This invention relates to a time base corrector for correcting fluctuations in a video signal transmitted through a transmission line or reproduced from a recording medium.
2. Description of the Prior Art
Such a conventional time base corrector is known that converts an input video signal by an A/D converter to a digital signal, records the digital signal into a memory operated by a write-in clock signal, reads the recorded signal from the memory by a standard clock, and converts the read-out signal back to its analog form. The write-in clock signal is arranged to synchronize with a horizontal synchronizing signal or a color burst signal carried in the input video signal and contains a phase shift equivalent to a time base error in either the horizontal synchronizing or color burst signal. The detection of the time base error in the horizontal synchronizing or color burst signal is executed during a horizontal scanning period. Accordingly, a time base error within one horizontal scanning period is no more corrected and remains as a velocity error.
For compensation for the velocity error, it is schemed that the read-out clock signal is phase modulated corresponding to a velocity error data fed from the write-in clock signal generating circuit and used for reading from the memory a desired data which is in turn converted to an analog form for output.
Such apparatus is disclosed in the form of a time base corrector in U.S. Pat. No. 4,165,524.
In this apparatus, the phase shift of the read-out clock signal is determined in proportion to a video data to be retrieved and hence, the video data has to be converted to an analog form immediately after read out from the memory with the read-out clock signal.
Accordingly, when a signal processing associated with time delays or continuation errors in the time base is conducted between the memory and the D/A converter, the timing between the video data to be D/A converted and the velocity error data to be used for phase modulation of the read-out clock signal will fail to keep up with. For example, a timing error will likely occur during the edge enhancement with the use of a line memory or reproduction of a still picture with field or frame memories. As the result, the velocity error can be no longer compensated but ironically enhanced.
Also, the processing of signals read from the memory through such an arithmetic operation as including in-line and in-field calculation involves computation of the video signals having different velocity errors. Hence, the velocity error itself becomes different from the one before the arithmetic operation. In addition to the timing error between the velocity error and the video data, another problem arises that the video data released from a calculating circuit fails to correspond with the velocity error data and thus, no exact compensation for the velocity error will be executed.